Adjustable backrest



FIG. 1 is a front perspective view of an adjustable backrest showing ournew design;

FIG. 2 is a front elevational view;

FIG. 3 is a rear elevational view;

FIG. 4 is a top plan view;

FIG. 5 is a bottom plan view; and,

FIG. 6 is a left side elevational view thereof.

The ornamental design for an adjustable backrest, as shown anddescribed.